1. Field of the Invention
This invention relates to hollow structures and, more particularly, to the manufacture of airfoils for producing a lightweight, high-strength hollow airfoil.
2. Description of the Prior Art
Hollow airfoils are utilized by gas turbine engine manufacturers to reduce the weight of the engine. Weight reduction becomes vitally important as the gas turbine engine thrust is increased. One of the ways that thrust is increased is by increasing engine size. As the engine size increases, individual part size and part weight also can increase. In the past, weight reduction has been accomplished by developing strong, light weight alloys. For airfoils such as compressor blades, which typically are solid for smaller engines, the increase in size by design precludes the use of solid airfoils because of the substantial weight gain, since the stresses on the disk caused by the rotating airfoils are unacceptably increased, even when light weight materials such as titanium alloys are used. In order to produce useful but light compressor blades without incurring unacceptable weight penalties, it is necessary to either manufacture composite blades or, in the alternative, hollow metallic blades.
Titanium alloy parts such as compressor blades frequently are formed by taking advantage of the superplastic forming and diffusion bonding behavior of certain metals. Superplastic forming is a technique that relies on the capability of certain metals, such as titanium alloys, to develop high tensile-elongation with a minimal tendency toward local necking when submitted to coordinated time-temperature-strain conditions within a limited range. Superplastic forming is useful in producing a wide variety of strong, lightweight articles.
Many of the same materials used in superplastic forming also can be diffusion bonded. Diffusion bonding is a process which forms a metallurgical bond between similar parts which are pressed together at an elevated temperature and pressure for a specific length of time. Bonding occurs by the diffusion of atoms across adjacent faces of the parts at elevated temperatures. Diffusion bonding provides substantial joint strength with little or no geometric distortion while substantially maintaining the physical and metallurgical properties of the bonded metal.
It has long been desirable to fabricate various aircraft components and turbomachinery components, such as door panels, wing flaps, blades and vanes, as hollow bodies. See, for example, U.S. Pat. Nos. 5,075,966; 4,364,160; 3,825,984; 3,383,093 and 3,220,697. See also U.S. Pat. No. 3,466,166 for hollow articles generally. The benefits of such hollow articles include a substantial reduction in weight which provides improved fuel efficiency and increased thrust-to-weight ratio. Despite the increasing popularity in applying diffusion bonding and superplastic forming techniques for manufacturing aircraft components, there are many critical problems to overcome in order to devise a process for successful manufacturing of hollow airfoils.
Parts formed using diffusion bonding and superplastic forming techniques can have very complex geometries, can exhibit highly nonlinear material behavior and are subject to large irreversible strains. Thus, there exists a possibility of many deformation-induced instabilities, such as necking, grooving, buckling and shear localization, which substantially weaken the structural integrity of the part.
The stringent requirements for both the external aerodynamic shape and internal structure of hollow airfoils present another problem in the manufacture of such parts. In order to produce the desired final shape and thickness, the in-process shape (i.e., the shape and size of the part prior to superplastic deformation) must be known.
Current processes utilize superplastic forming and diffusion bonding to form hollow articles such as compressor airfoils. One such process is described in U.S. Pat. No. 3,628,226. The process comprises grooving flat blanks and forming the grooved blank into a preliminary airfoil surface without longitudinal curvature. Next the blanks are machined flat on the inner surface. The blanks are then diffusion bonded to form a rough airfoil. The rough airfoil is then twisted to form a final airfoil. The infirmity with this process is that the twisting operation, which occurs after the diffusion bonding operation, may not produce a part having the desired final internal shape or skin thickness. The twisting operation itself, performed at the elevated temperatures associated with superplastic forming, may produce unacceptable necking, locally reducing part thickness below the minimum value required. To achieve the necessary final shape, additional machining may be required, and this machining may reduce the hollow part thickness below the minimum allowable thickness, resulting in a high scrap rate and accompanying higher costs.
Another method is described in U.S. Pat. No. 5,063,622. By use of this method, a shroudless blade which is also hollow is produced. This process requires the forming of a hollow, untwisted blade of lenticular cross-section with axially-extending ribs connecting opposite walls of the blade. The blank for each half of the blade is formed to establish the lenticular cross-section shape. After the blade is formed with the curvature in the cross-sectional direction, the concave side is cut flat and grooves are machined therein leaving lands which will form the blade ribs. The blade halves are bonded together. The untreated blade is then creep-deformed. Then, superplastic forming is used to establish the final form of the blade. The hourglass shape of the ribs with this minimum cross section at the diffusion bond plane results in the highest compressive stresses at the ribs. A substantial problem of skin buckling exists due to the combination of minimum cross-section and high compressive stresses. Furthermore, because the stresses are high in the rib region, the stresses in the surrounding leading and trailing edge regions are low, leading to a higher probability of disbonds in these regions.
Still, this method represents an improvement, but requires the use of dies for hard die pressing. These dies are subject to wear and require frequent refurbishment or replacement. The die pressing operation can thus be subject to extensive out-of-tolerance conditions. These out-of-tolerance conditions, even when small, can result in the failure of the land areas to match up in the diffusion bonding step, which can lead to inadequate diffusion bonding and premature failure, or if timely detected, scrappage of the part. Furthermore, forming may result in local buckling due to the widely varying metal thicknesses. Also contributing to scrappage are the machining operations in step 5 of U.S. Pat. No. 5,063,622, which can lead to scrappage after diffusion bonding if the minimum thickness is not maintained. Alternatively, to assure against such a problem, additional material can be accommodated into the blade to assure that sufficient stock is present. Of course, this is undesirable since additional weight unnecessarily may be added to the parts, if insufficient metal is removed, or conversely, the part may be too thin to meet design requirements if too much metal is removed. Finally, it must be recognized that the configuration of the blades at this point in the manufacturing sequence makes accurate measurement and control of blade wall (skin) thickness very difficult, so that existing inspection techniques may not detect out-of-tolerance conditions.
Therefore, it is an object of the present invention to provide a method for manufacturing by superplastic forming and diffusion bonding, hollow airfoils having aerodynamic shapes and very complex geometries to the final desired shape and thickness without compromising the physical and metallurgical properties of the bonded metal by achieving the necessary diffusion bond in the skin region while alleviating problems of (skin or rib) buckling.